Some source systems store their fact tables per year and thus create a new fact table every year. In Belgium there's an accounting product Venice from Unit 4 that works this way.
The structure of these tables is identical but when you want to add a "younger" or "new" systems that don't have fact tables for the previous years, they can't be added as an additional connection.
It would be handy to mark an additional connection or even an additional table as "allowed to be missing" so that DH continues its execution.
This would also be handy for Navision-based where some versions have extra tables that older versions don't have, but the customer hasn't upgraded all its systems yet.
A worldwide brewery comes to mind in this case.